System for positioning movable members with absolute dimension with selectable offset point

ABSTRACT

A positioning system in which the actual position of a controlled member is always expressed in absolute dimension with respect to a fixed reference point and its position with respect to a second selectable reference or offset point is determined by subtracting the absolute value of the second reference point from the actual position of the controlled member.

United States Patent Inventor Geoffrey A. Ross Canton Center, Conn.

Appl. No. 818,878

Filed Apr. 24, 1969 Patented July 13, 1971 Assignee Pratt 81 WhitneyInc.

West Hartford, Conn.

SYSTEM FOR POSITIONING MOVABLE MEMBERS WITH ABSOLUTE DIMENSION WITHseuacunu: orr-"srzr rom'r 1o Claim, 10 Drawing Figs.

u.s. C1 318/572, 3 18/594 1.1:. CI ..G05b 1111s Field 01 Search ..318120.1 10,

Primary Examiner- Benjamin Dobeck Auorney- Delio and MontgomeryABSTRACT: A positioning system in which the actual position of acontrolled member is always expressed in absolute dimension with respectto a fixed reference point and its position with respect to a secondselectable reference or offset point is determined by subtracting theabsolute value 01' the second reference point from the actual positionof the controlled member.

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SHEET 6 OF 6 FROM \0' 5 Demos RELFW RELFIY INVENTOR co-firm 9. Rossmafia A Wm SYSTEM FOR POSITIONING MOVABLE MEMBERS WI'I'II ABSOLUTEDIMENSION WITH SELEC'I'AILE OFFSET POINT This invention relates topositioning systems for machine tools, and more particularly relates toan absolute positioning system.

The present invention is particularly adapted for use in jig borerswhere very precise and accurate positioning is required.

In some machine tool-positioning systems a quantizer, which may be inthe form of a rotating disc, provides pulses representative of movement.The pulses are used in conjunctionwith a position register to store theposition of a machine part with respect to a given axis. Anotherposition-indicating system is a device known as a resolver whichcompares through phase relations an actual position with a commandedposition to provide an error.

Such systems work satisfactorily and are widely used. However, suchsystems are not absolute in the sense that they will remember the actualposition of a machine part, such as a slide or saddle, when power isremoved therefrom. For example, in a position register using activeelements, such as transistors or other electronic switches, if powershould be removed, the electronic switches are deenergised and theposition of the machine part is lost. Accordingly, when a large piece ofwork is being acted upon and requires many operations, it is necessaryto leave the control power on continuously whether the machine is inoperation or not.

The present invention provides a new and improved control system inwhich the position of a slide may always be recognieed as soon ascontrol power applied to the, control systerm'l'he present inventionfurther provides a positioning system which operates on an absolutesystem utilizing only two coordinate axes extending in one directionfrom a reference zero point and in which any given zero point may beselectively placed in the system to establish a reference point at anylocation on the workpiece within the range of the machine. The inventionenhances the very precbe positioning required in a jig borer andcontrols the velocity of the machine slides as afunction of the existingerrorto provide a wide speed range and to rapidly and-precisely bringthe machine slides to a commanded position.

An object of this invention is to provide a new and improved positioningsystem for a machine tool.

Another object of this invention is to provide a machinetool-positioning system having new and improved meanafor continuouslyrecognizing the position of the machin'eslides.

Another object of this invention is to provide machine toolpositioningsystem having new and improved means for predetermining a referenceoffset point.

A further object of this invention is to provide new and improvedvelocity control arrangement in a machine tool-positioning system.

The features of the invention which are believed to benovel are pointedout with particularity and distinctly claimed in the concluding portionof this specification. However the invention both as to its operationand. organization, and together with further objects and advantagesthereof may best be appreciated by reference to the following detaileddescription taken in conjunction with the drawings. in which: .4.

FIG. 1 is a block diagram of the overall system embodying the invention;

FIG. la is a graphical representation of the axes along which thecontrolled members of the machine may operate;

FIGS. 20 and 2b graphically represent timing signal waveforms;

FIG. 3 is a drawing of a drive system for the controlled members;

FIG. 4 is a diagram in logical schematic form of theerror interpreter ofFIG. 1;

FIG. 5 is a diagram in lcgicalschematic form of the controls for thedrive system of FIG. 3;

FIG. 6 is a diagram in logical schematic form of a selection networkutilized to determine the axes of movement for which numericalinformation will be interpolated;

FIG. 7 is a diagram in logical schematic form exemplifying the manner inwhich the speed of the drive system is controlled in response to theinterpreted magnitude oferror; and

FIG. 8 is a diagram in logical schematic form exemplifying a part ofFIG. 7 in more detail.

Referring now to the drawings. The system 10 generally comprises adecoder and information distributor I I, also known as a transistor. Thedecoder and information distributor ll may receive inputs from punchedtape to a tape reader 12 orfrom a manual keyboard data input 13.

The decoder and information distributor applies the X-position andY-position commands in binary coded decimal (BCD) forms to X-commandregister 14 and Y-command register I5. Registers 14 and 15 in the formof the invention illustrated are seven decade 8CD static registers whichare sampled least significant decade first by marker signals M -MDecoder ll further distributes information to a G function (it cyclecontrol storage network 16.

The machine tool may be exemplified as having a table I? and a carriageI8 movable along an X--\' axis as shown in FIG. la. The table isprovided with a drive I9 and the carriage with a drive 20. Carried onthe table and carriage are precision glass scales 21 and 22,respectively, each having a stationary reading head 23 and 24,respectively. The reading heads 23 and 24 will read the absolutepositions on scales II and 22 with respect to a zero point along the X-and Y-axc's, as exemplified in FIG. la. Associated with the scales andreading heads is an interpolation system 25. As exemplified thisposition readout system is an instrument called a DIG produced byGeneral Measurement Research, Inc. and distributed by Automatic Gauges,lnc., Rochester, N.Y. The BIG consists of three elements, a precisionglam scale, a compact reading head, and

interpolation means for providing signals in a BCD format. As

shown, the DIG head and scale are mounted on separate machine elementsmoving in relation to one another. The reading head optically scans thescale and makes absolute determination of position relative to a zeroreference point on the scale This information is applied to theinterpolation system 25 which provides a seven decade digit BCD numberfor each reading head. This position information is selectively appliedthrough X-Y gate 26 to a summer or subtractor 21A: herein described, therange is 99.99999 inches, at seven decade number. v

As will hereinafter be described, the sequencing of the seven 8CD digitsare under the control of marker signals M,- M. which form one markercycle. Other control signalsare derived from the cycle control network.Various clock signals are also utilized which are designated C1-C4 andfour such clock signals comprise a clock cycle. Etch marker has aduration of one clock cycle. Reference to FIG. in will show the form ofthe clock cycles and FIG. 2b exemplifies the relation of the markersignals thereto.

The clock signals are generated by a network as shown in the 0.5. Pat.No. 3,417,303'of Johann Reuteler, assigned to the same assignee as thepresent invention. The markers are generated through use of aseven-stage shitt register (not shown) which is shifted ea'ch clockcycle to successively produce amarlter signal at each stage in a mannerwell known to those skilled in the art. As will hereinalter be madeapparent, the clock signals C2 and C4 are generally used for gating andthe signals Cl and Cd are used for resetting or setting purposes. Thelogic herein illustrated is of the NOR type, is exemplified in said US.Pat. No. 3,4 I 7,303.

The system further provides the function of providing a predeterminedoffset as a reference point for operation as, for example. the pointX-S, Y-4, as shown in FIG. Ia. This may be accomplished for each axis byan X-ollset register 28 and a Y-offset register 29. The offset devicesare conventional converters which will convert a dial reading in decimalarabic form to a binary coded decimal number which, in this case, is aseven digit number. A suitable register is one termed a Digi- Switch andis manufactured by Digitran Co. of Pasadena California. These numbersare set and stored in registers 28 and 29 and may be read out in theproper sequence during markers M M when X-Y selection gate 30 is opened.

in this regard, gates 26 and 30 are opened in synchronism so as to passthe X- or Y-numbers to summer 27. Summer 27 algebraically adds the X-and Y-offset coordinates to the number read from the DIG scale. Theofi'set numbers are applied to summer 27 as negative numbers. Summer 27comprises a seven decade binary coded decimal bidirectional adder havingstages 2'-2"' and the corresponding BCD decades are applied thereto fromthrough gates 26 and 30. Gates 26 and 3-0 are selectively opened to passeither X- or Y- digits as is the gate 3!. Gate 3] will selectively passthe X commanded position or the Y-commancled position. Gates 26 and 30will operate to pass the X-digits at the same time or the Y-digits atthe same time. The output of summer 27 is read out serially as binarycoded decades with marker signals M M,,. This provides the offsetposition from the zero axis. This, in essence, gives an indication ofwhere the table and carriage are located with respect to the offsetcoordinates rather than to the zero point, and thus allows theestablishment of any reference point for positioning purposes. Thecommanded X- position and commanded Y-position are then compared withthe offset position in a summer 32. The seven BCD digits are applieddecade by decade, least significant decade first, to summer 32 fromeither register 14 or register through gate 31 in timed relation withthe output of summer 27. The output of summer 32 thus indicates theposition error between the commanded position and the actual position asoffset. Summer 32 includes sign means 330 for detecting a borrow orcarry in the most significant position to indicate the direction ofmovement.

The magnitude of the position error which indicates the error betweenthe commanded position and the offset actual position is applied to anerror interpreter 33 which determines the magnitude of the error and sosignifies to cycle control network 16.

in response to the magnitude of the error, the cycle control network 16may command the table and/or carriage drive to traverse at a high rate,traverse at a medium rate, or place the traverse speed under the controlof a digital-to-analog converter which controls the drive at a rateproportional to the magnitude of the error. The numerical position errorin BCD form may be applied to digital-to-analog converters 34 and 35 forthe X- and Y-axes, respectively, to drive the table and carriage at arate proportional to the magnitude of the position errors when the erroris in a predetermined range. As exemplified herein, the errorinterpreter will detect five errors or error ranges, hereinafterreferred to as E,, E E E and E For purposes of illustration, theseerrors will be considered as follows:

Each slide (table or carriage) is designated by the reference numeral 40(FIG. 3) and is driven by a lead screw 41 which may be rotated at arapid traverse rate by an AC motor 42 under control of a motorcontroller 43. When the error interpreter determines that the error hasdecreased to range 5,. the AC line contactors are opened and a DC motor44 drives lead screw 41 through a two-speed gearbox 45 which includesclutches CLl and CL2. When clutch CL! is energized, gearbox 45 isconnected for fast speed operation. Clutch CLl is deenergized and clutchCLZ energized when the error has further decreased to a predeterminedvalue as exemplified by error E When the error has decreased to E themotor controller 46 is placed under control of the digital-to-analogconverter output voltages which are proportional to the existingposition error. Then when the error decreases to point E energization ofthe low speed clutch is changed and the slide velocity is furtherdecreased. Still further, when the position error decreases to E asignal indicative thereof is applied to motor controller 46 so that thelead screw 41 is rotated at a much lower creep feed until the positionerror is reduced to zero.

The error interpreter is exemplified in schematic logical form in FIG.4. The position error summer 32 is exemplified as a four-bit register.if at marker times M and M, which correspond to the tens and unit digitof the error, respectively, there is decimal l or greater in summer 32,this will be detected by gate 48, and gate 48 will set memory 49. Whenmemory 49 is set it indicates that at the last comparison of the offsetposition and the command, the error was greater than one.

Memory 49 will be resetjust prior to the next M marker at marker M; andclock pulse (:3 through gate 50.

In a similar manner, gates 51, 52 and S3 will detect if the number insummer 32 is 5 or greater at marker time M; which corresponds to thetenths decade. if any of gates 51, S2 or 53 detects that at M, thenumber in register 32 is 5 or greater, gate 54 will set memory 56, atmarker W, through inversion gate 55.

When memory 56 is set it indicates that the error is equal to or greaterthan 0.5 inches. Memory 56 is reset at marker time M at clock C3. Gate57 will indicate when the error is l.5 inches or greater. Gate 58through inversion gate 59 will detect when the error is less than l.5inches and greater than or equal to 0.5 inches. Gate 60 will indicatewhen the error is less than 0.50 inches.

in a similar manner which is well known to one skilled in the art, thehundredths and thousandths decades of the error may be sampled at markertime M and M, to detect when the error reaches 0.0l3 inches. Similarly,the ten-thousandths decade may be sampled at marker times M, and M todetermine when the error decreases to 0.0004 inches and the drive is toshift to creep speed.

Reference is now made to FIG. 5 which exemplifies the motor controls. inthe final positioning of two slides, it is desired to bring one axis tozero error or to the set point and then move the other axis to its setpoint. A gate 61 is provided which detects when the Y-error E,- is lessthan 0.0l3 inches and the X-error is not zero, then gate 61 emits asignal which states wait for X" to a Y-axis move memory 62. When memory62 is set it will apply an inhibiting signal to gates 63 and 64 which,as will hereinafter be made apparent, will disabIe the Y-axis drive.Y-axis move memory 62 will also be set by a Y-set or Y-error equals zerosignal and a signal from cycle memory 65 signifying that both X- andY-slides are at the set point. At such time, cycle memory 65 will alsoapply a cycle complete signal to cycle control network 16. An X-movememory 66 will sense that an error exist" in the X-axis and responsethereto will apply enabling signals to gates 67 and 68. When gate 67receives a signal from the X-axis move memory 66 and also a signal fromerror interpreter 33 that an X-axis error exists and the error is 5,,gate 67 will enable a relay driver 69 which will pick up a motor controlrelay and close the line to AC motor controller 46 thus permitting rapidtraverse along the X-axis. A directional sign error derived fromregister 32 is further applied to motor controller 46 to determine thedirection in which the X-slide will be moved at rapid traverse.

The operation of the Y-axis slide in rapid traverse is identical andneed not be explained in detail.

If the error in the X-axis is E, such conditions are detected by gate 68which will a ply an indicative signal to motor controller 46 which willdrive DC motor 44 at the fast traverse speed.

The output of X-move memory 66 is applied to a medium speed clutchcontrol network 0 for clutch CL] and to slow speed clutch controlnetwork 7] for clutch CL2. The clutch control networks are merely theconventional clutch energizing coils and associated circuitry therewith.So long as the error is in either the E, or E range, control 70 willenergize clutch CLl for operation at fast traverse and in the highdigital-to-analog range. These are signified by either the E, or

E signals. When an E, or E signal is not received by clutch controlnetwork 70. clutch CLl will be deenergized. Clutch CL2 is energized fromclutch control 71 when the X-move memory 66 is set and the error is inrange E, or E The difference in the speeds provided by the gearbox is onthe order of l0:l.

It will be noted that the interpolation system 25 performs the positioninterpolation for both axes. This requires a finite time. Accordingly.the overall position interpolation system alternately calculates theX-axis position error and the Yaxis position error. and applies theappropriate X- and Y-signals to interpolation system 25 and gates 26, 30and 31. This alternate selection system is located in cycle controlnetwork 16 and may comprise a timing network in the form ofa two-stagecounter 74 (FIG. 6) which counts the M marker pulses and develops anoverflow pulse every four marker cycles. These overflow pulses areapplied to r" as 75 and 76. each of which is adapted to set a memory 77and 78, respectively. The network of FIG. 6 is arranged to rememberwhich axis is being acted upon and at the next switching time to enableposition interpolation for the other axes. Assume that the Y-axis interpolation is taking place which is exemplified by memory 78 being set.When memory 78 is set, it resets X-memory 77. When counter 74 againoverflows it will apply signals to gates 75 and 76. However. gate 76 isinhibited by the set condition of memory 78, but gate 75 is enabled dueto the reset condition of memory 77. Therefore. memory 77 will be setand simultaneously will reset memory 78.

lnterpolator 25 receives a read X or read Y from memory 77 or 78 andupon receipt of such signal applies a signal over line 25a to inhibitgates 75 and 76 to prevent resetting of memories 77 and 78 until theconversion of the scale reading to BCD is complete. When such conversionis complete, an enabling signal is applied to gates 79 and 80. Thesegates then pass a signal to memories 790 or 80a. Memories 79a and 80asupply the X- and Y-control signals to gates 26, 30 and 31. When one ofmemories 790 or 800 is set it resets the other and they will remain insuch condition until the other of gates 79 or 80 passes another settingsignal.

This will not occur until counter 74 again overflows. or anotherspecified time delay occurs. When the disabling signal applied to gates75 and 76 by interpolator 25 is removed. memories 77 and 78 may changestate. However, the state of memories 79a and 800 will not change untilinterpolator 25 completes its position calculations and enables gates 79and 80.

The position interpolation time of interpolator 25 may be two or threemarker cycles. Therefore. the time of overflow of counter 74 is made asufficient number of marker cycles for summer 32 to calculate the newposition error.

As the decades of the error are determined by summer 32. the l0. l0",l0", l0" decades in BCD are applied to fourbit binary registers 81. 82,83 and 84, FIG. 7. For each axis a BCD number is thereafter converted toanalog form as, for example. for the X-axis by digital-to-anrtlogconverter 34, and are applied through range shifters 85. 86 and 87. Theconverted analog voltages are derived from only three of the decades ofthe error. In the high digital-to-analog range. the 10", l0", and l0decades are converted to an analog voltage and thereafter applied tomotor controller 46. During the low range of D/A operation. when theerror is in the E, range. the number appearing in the l0". l0 andIO'decades are converted to an analog voltage and applied to motorcontroller 46.

The operation of one of the range shifters 85 with respect to the l0 andI0" decades the X-axis and the digital-to-analog converter is shown inFIG. 8. The 10 decade register 81 during the high range ofdigital-to-analog control operation is sampled at marker M, when theX-error is in the E; range by gates 90. 91, 92 and 93. When an outputappears. they are transmitted through gates 94. 95. 96 and 97 so long asX-move memory 66 is set. When a signal is passed by gates 94-97 it setsa corresponding memory 98, 99. 100 and 101, respectively. When thememories -101 are set they will pick up a relay 102. 103. 104 and 105.respectively, to connect the appropriate resistance arms in circuit inthe X-axis digital-toanalog converter.

When the position error has decreased to the E, range gates 90. 91, 92and 93 are disabled and gates 106. 107. 108 and 109 pass the BCD of thel0" decade.

This will detect the 10" decade at marker M, when the error is in the Erange. In this manner. the range of the digitalto-analog converter isshifted one decade when the error range changes from E to 5,. When thisoccurs. medium speed clutch CL] is deenergized and slow speed clutch CL2is energized.

With this arrangement the speed or velocity of the slides is controlledin accordance with the existing position error. In this manner. theslide position approaches its commanded end or set point with decreasingvelocity and is precisely posi tioned in a minimum time.

The provision of the various speed ranges and in particular the high andlow digital-to-analog converter in conjunction with the two-speedgearbox more than doubles the range of the DC motor control and furtherincreases the range of the digital-to-analog corverters one decade.

It may thus be seen that the objects of the invention set forth above aswell as those made apparent from the preceding description areefficiently attained. A preferred embodiment of the invention has beenset forth for purposes of disclosure. However, other embodiments of theinvention as well as modifications to the disclosed embodiment may occurto those skilled in the art. Accordingly. the appended claims areintended to cover all embodiments and modifications of embodiment to thedisclosed invention which do not depart from the spirit and scopethereof.

What I claim is:

1. A system for positioning two or more movable members along two morerespective axes. comprising means providing numerical signals indicativeof a commanded position of each member, means for reading the absoluteposition of each member with respect to its axis from a reference point,means responsive to said reading means for providing numerical signalsindicative of said actual positions. means for establishing numericalsignals indicative of ofi'set positions of said m bers. first means forderiving numerical signals indicative of the difference between saidabsolute positions and said offset positions to derive a numericalposition offset signal. second means for deriving numerical signalsindicative of the difference in said position ofiset signal and saidnumerical command position signals to provide position error signals,and means for moving said members responsive to said position errorsignals.

2. The system of claim 1 wherein said first means alternately determinesthe numerical position offset signal for each axis. and means foralternately applying the actual position signals and the offset positionsignals for each axis thereto.

3. The system of claim 1, further including means for determining themagnitude of the position error of each member. and moving each memberat a velocity which is a function of its position error.

4. The system of claim 1 wherein said first and second means are enabledto derive said differences periodically for each axis.

5. The system of claim 1 wherein said numerical signals indicative ofsaid actual positions are applied to said first means as absolutenumbers and said numerical signals indicative of said offset positionsare applied thereto as negative numbers.

6. The system of claim 1 further including means for moving said membersas a function of the magnitude of said position errors.

7. The system of claim 1 further including digital-to-analog convertersfor each axis adapted to connect said command position signals to analogvoltages, a DC motor for moving each of said members, and means fordriving said DC motors at speeds proportional to said analog voltages.

8. A drive system for moving a member at varying speed to a commandedposition. comprising a driving member, means drivingly coupling saidmember to said driving member, means for determining the position errorbetween the commanded position of the member and the actual positionthereof and representing said error in digital form, a twospeed gearboxhaving clutches energizable to determine a high speed and a low speed. aDC motor for driving said driving member through said gearbox, controlmeans for controlling the speed of said DC motor in response to themagnitude ofa control signal applied thereto, a digital-tdanalogconverter, said converter being constructed and arranged to convertFirst and second predetermined weights of the error in digital form toan analog control signal, means for determining the mag nitude of theposition error, first means responsive to a predetermined magnitude ofthe position error for deenergizing said high speed clutch andenergizing said low speed clutch, and second means responsive to saidpredetermined position error for shifting the weight of the digitalnumber applied to said converter.

9, The system of claim 8 further including another motor for drivingsaid driving member at a fixed rapid speed when said position error isabove a second predetermined value greater than said predeterminedmagnitude.

H), The system of claim 8 further including means for applying aconstant level speed control signal to said motor controller to drivesaid motor at a fixed fast speed when said position error is above asecond predetermined value greater than said predetermined magnitudevUNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 593O91 Dated July 13 1 971 Geoffrey A. Ross Inventor(s) It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 1 line 45 after provide" insert a Column 2 line 19 "M-M" shouldread M M Column 3, the Table should appear as shown below:

@3335 Ran e, in. Slide Speed E14 1.5, Rapid E2 1.5, 20.50 Fast E3 0.50,2.0.013 High D/A ELI 0 .013, 20.000 4 Low D/A E5 0.00, 0.0OO I CreepColumn 5, line 52 "10 10 10 10 should read Column 5, line 59, "10 10 10should read PC4050 USCOMM-DC scan-P09 9 U S, GDVERNMENY PHINTING OFFICEI059 0-!6-lll,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 593091 Dated July 13 1971 Geoffrey A. Ross PAGE 5 Z Inventor(s) It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 5 line 62 "10 10 and 10 should read Column 5 line 66 "10 10should read --10- 10- Column 5 line 67 "10 should read --1o- Column 6line 7 "10 should read --l0"' Claim 1, line 2 after "two" insert -or'-.

Signed and sealed this 3rd day of October 1972.

(SEAL) Attest:

EDWARD M. FLETCHER ,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents USCOMM- DC 608764 09 0 u s GOV zmmzm mum-ma orrlczan o-au-au

1. A system for positioning two or more movable members along two morerespective axes, comprising means providing numerical signals indicativeof a commanded position of each member, means for reading the absoluteposition of each member with respect to its axis from a reference point,means responsive to said reading means for providing numerical signalsindicative of said actual positions, means for establishing numericalsignals indicative of offset positions of said members, first means forderiving numerical signals indicative of the difference between saidabsolute positions and said offset positions to derive a numericalposition offset signal, second means for deriving numerical signalsindicative of the difference in said position offset signal and saidnumerical command position signals to provide position error signals,and means for moving said members responsive to said position errorsignals.
 2. The system of claim 1 wherein said first means alternatelydetermines the numerical position offset signal for each axis, and meansfor alternately applying the actual position signals and the offsetposition signals for each axis thereto.
 3. The system of claim 1,further including means for determining the magnitude of the positionerror of each member, and moving each member at a velocity which is afunction of its position error.
 4. The system of claim 1 wherein saidfirst and second means are enabled to derive said differencesperiodically for each axis.
 5. The system of claim 1 wherein saidnumerical signals indicative of said actual positions are applied tosaid first means as absolute numbers and said numerical signalsindicative of said offset positions are applied thereto as negativenumbers.
 6. The system of claim 1 further including means for movingsaid members as a function of the magnitude of said position errors. 7.The system of claim 1 further including digital-to-analog converters foreach axis adapted to connect said command position signals to analogvoltages, a DC motor for moving each of said members, and means fordriving said DC motors at speeds proportional to said analog voltages.8. A drive system for moving a member at varying speed to a commandedposition, comprising a driving member, means drivingly coupling saidmember to said driving member, means for determining the position errorbetween the commanded position of the member and the actual positionthereof and representing said error in digital form, a two-speed gearboxhaving clutches energizable to determine a high speed and a low speed, aDC motor for driving said driving member through said gearbox, controlmeans for controlling the speed of said DC motor in response to themagnitude of a control signal applied thereto, a digital-to-analogconverter, said converter being constructed and arranged to convertfirst and second predetermined weights of the error in digital form toan analog control signal, means for determining the magnitude of theposition error, first means responsive to a predetermined magnitude ofthe position error for deenergizing said high speed clutch andenergizing said low speed clutch, and second means responsive to saidpredetermined position error for shifting the weight of the digitalnumber applied to said converter.
 9. The system of claim 8 furtherincluding another motor for driving said driving member at a fixed rapidspeed when said position error is above a second predetermined valuegreater than said predetermined magnitude.
 10. The system of claim 8further including means for applying a constant level speed controlsignal to said moTor controller to drive said motor at a fixed fastspeed when said position error is above a second predetermined valuegreater than said predetermined magnitude.